Method for monitoring chloride content in porous materials

Abstract

The present invention relates to a method for monitoring chloride content in porous materials, preferably in reinforced concrete structures, in which a voltammetric sensor comprising a metal electrode is used, characterised in that the electrode metal forms poorly soluble ionic compounds with chlorides and a potentiodynamic excitation signal is applied to the electrode, causing an oxidation-reduction process to take place that favours the formation of the poorly soluble ionic compounds. The formation of the poorly soluble ionic compounds produces a response in the form of electric current intensity that is related to the chloride content in the monitored material.

Description

[0001]DESCRIPTION OF A PROCEDURE FOR MONITORING CHLORIDE CONTENT IN POROUS MATERIALS TECHNICAL FIELD The present invention relates to the field of systems for monitoring chloride content in porous materials, especially in reinforced concrete structures, and more specifically to a procedure for monitoring chloride content. BACKGROUND OF THE INVENTION Effective monitoring of chloride presence in porous materials is important because it affects the productivity of agricultural soils and the durability of reinforced concrete structures (RCS) subjected to marine environments and de-icing salts. On the one hand, anthropogenic activities cause an excessive accumulation of chloride anion (Cl-) in soils, either through the excessive use of fertilizers or through atmospheric deposition from industrial sources. The overabundance of Cl- in soils can reduce their fertility and cause toxicity in crops.From the perspective of reinforced concrete structures (RCS), depassivation of the reinforcement due to the presence of chlorides is one of the most common causes of failure in RCS. This phenomenon particularly affects large structures located in marine environments, such as bridges, breakwaters, and offshore platforms, as well as road bridges exposed to de-icing salt. The degradation of these structures entails very costly repair work and can also affect the safety of both people and the environment. Furthermore, premature repair or demolition of these large concrete structures generates excessive resource consumption and waste production, jeopardizing the sustainable development of our societies.Therefore, the detection, estimation, and prediction of chloride presence in soils and concrete structures (CS) are current challenges in the development of smart agriculture and in monitoring the lifespan of structures and the sustainable development of our society. For this reason, the detection, quantification, and prediction of chloride presence in CS has been studied for many years, using both destructive (laboratory tests) and non-destructive techniques. The latter show more promising results, as they allow for real-time monitoring of chloride presence, enabling the development of estimation and prediction models that allow for optimal intervention of the structure—that is, minimizing economic costs and material resources while maximizing the structure's lifespan.The most commonly used techniques are destructive and based on potentiometric and Volhard methods. They involve determining the amount of free or total chlorides in concrete samples taken from the structure. These methods are typically slow and expensive and do not allow for continuous, real-time monitoring of chloride levels in concrete. On the other hand, in recent decades, various non-destructive techniques have been developed for determining chloride levels in concrete, for example: Zhang Z, Hu J, Ma Y, Wang Y, Huang H, Zhang Z, et al. “A state-of-the-art review on Ag / AgCl ion-selective electrode used for non-destructive chloride detection in concrete.” Compos Part B Eng [Internet]. 2020;200(August):108289 o Torres-Luque M, Bastidas-Arteaga E, Schoefs F, Sánchez-Silva M, Osma JF "Non-destructive methods for measuring chloride ingress into concrete: State-of-the-art and future challenges. Constr Build Mater [Internet].2014;68:68–81. These techniques can be classified according to the nature of the measurement signal as electrochemical, optical, and other types of electromagnetic signals (radio frequency, laser, etc.). Most of these methods are highly sensitive to changes in pH, humidity, or temperature, which can have negative effects that reduce the precision and reliability of the measurements obtained. Among electrochemical signal systems, the most researched and widely used in EHA, due to the simplicity of the measurement system, are embedded potentiometric sensor systems. Among these, those manufactured using Ag stand out due to its high reactivity with chlorides, for example, those described in the following document: Tian Y, Zhang P, Zhao K, Du Z, Zhao T. “Application of Ag / AgCl Sensor for Chloride Monitoring of Mortar under Dry-Wet Cycles.” Sensors [Internet].2020 Mar 4;20(5):1394 o Jin M, Ma Y, Zeng H, Liu J, Jiang L, Yang G, et al.“Developing a Multi-Element Sensor to Non-Destructively Monitor Several Fundamental Parameters Related to Concrete Durability.” Sensors [Internet]. 2020 Sep 30;20(19):5607. In potentiometric systems used in high-impedance concrete (HIC), the chloride ion-selective electrode (EID) is usually made of Ag coated with AgCl (Ag / AgCl), because these sensors have a Nernstian response to variations in chloride and silver activity. Studies indicate that good results are obtained with this type of sensor even four years after being embedded in concrete. The measurement system manufactured with this type of sensor consists of Ag / AgCl ion-selective sensors (ISEs) and a reference electrode (Ref), connected to the ends of a high-impedance voltmeter. The ISE is embedded in the concrete and the Ref may or may not be embedded.The voltmeter measures the standard equilibrium potential difference over time between the ISE and the Ref, so that it is related to the activity ratio of Cl- and Ag. +This allows for the quantification of chloride concentrations in the pore solution of concrete. However, the functionality of these sensors for quantifying Cl- has limitations, such as their response being highly affected by both temperature and the adsorption of other analytes on the sensor surface. They require frequent calibration and are not good at detecting very low concentrations of the analyte of interest due to interference from the OH- anion, which is usually found at very high concentrations (generally on the order of 0.1 mol / liter). Furthermore, the reference electrodes used in the system typically have a shorter lifespan than the structure being monitored. More recently, electrochemical sensors based on the application of Impedance Spectroscopy techniques have begun to be developed, such as the one defined in the patent by Torres Luque MM et al.ES2683618T3, and the sensor developed by Huang, B. et al. “Sensors and Actuators^: B. Chemical Hybrid cement composite-based sensor for in-situ chloride monitoring in concrete structures.” Sensors Actuators B Chem [Internet]. 2023;385(March):133638. This type of system consists of two electrodes between which an alternating current signal is applied, the frequency of which changes over time. Results found in the literature show that this type of sensor measures changes in chloride concentration with good sensitivity. However, it still exhibits interference from pH and temperature variations. Furthermore, the type of excitation signal is complex from an operational standpoint for electronic equipment, which can significantly hinder its implementation in real-world structures.Therefore, considering the limitations of current technologies, developing a system for detecting, estimating, and predicting the presence of chlorides in aqueous solutions remains a challenge. Voltammetric sensors are widely used in various fields related to the characterization of systems in solution and quality control, such as in food technology or for monitoring wastewater treatment processes, but this technology has been almost entirely unexploited in non-aqueous media, such as concrete.The application of voltammetric sensors in a porous medium such as concrete presents certain difficulties, but it is promising because it can allow the detection and quantification of agents that affect the durability of reinforced concrete structures (RCS), as well as the development of control and prediction models related to their deterioration. These models are more accurate than those developed with existing monitoring systems. EXPLANATION OF THE INVENTION The present invention is based on the application of voltammetric techniques in systems for monitoring reinforced concrete structures. An electrochemical sensor has been developed, applicable for use in porous materials such as concrete and mortar. In particular, a method has been developed capable of monitoring the chloride content in these media when the electrochemical sensor is embedded within them.The sensor measures the chloride content of concrete, but it can also be used in other media such as soil, chemical storage tanks, wastewater, biological media, and similar materials. The present invention relates to a method for monitoring chloride content in porous materials, preferably reinforced concrete structures. This method uses a voltammetric sensor comprising a metal electrode, the metal of which forms sparingly soluble ionic compounds with chlorides. A potentiodynamic excitation signal is applied to the electrode, inducing an oxidation-reduction process that promotes the formation of these sparingly soluble ionic compounds. The formation of these sparingly soluble ionic compounds produces an electrical current intensity response that is related to the chloride content in the monitored material.Preferably, the applied potentiodynamic excitation signal is rectangular or triangular in shape. Preferred embodiments of the present invention are described in the dependent claims. Throughout the description and claims, the word "comprises" and its variants are not intended to exclude other technical features, additives, components, or steps. Furthermore, the word "comprises" includes the case "consists of." For those skilled in the art, other objects, advantages, and features of the invention will become apparent partly from the description and partly from the practice of the invention. The following examples are provided by way of illustration and are not intended to be limiting of the present invention. Moreover, the present invention covers all possible combinations of the embodiments indicated herein.BRIEF DESCRIPTION OF THE DRAWINGS To complement the description provided and to aid in a better understanding of the invention's features, a set of drawings is included as an integral part of this description. These drawings, for illustrative purposes only and not intended to be limiting, depict the following: Figure 1A – Triangular potential excitation signal. Figure 1B – Graph representing the normalized electrical current response of the working electrode surface to the applied potential difference (voltage). Figure 2 – Rectangular waveform. Figure 3A – Single-sensor configuration. Figure 3B – Multi-sensor configuration. Figure 4 – Response of the sensor embedded in concrete. Accumulated charge: Solid line – concrete without chlorides. Dashed line – concrete with chlorides. Figure 5 – Response of the sensor embedded in concrete.Electric current density. Solid line - concrete without chlorides. Dashed line - concrete with chlorides. Figure 6 - Response of the Ag sensor embedded in concrete: Solid line - concrete without chlorides. Dashed line - concrete with chlorides. Figure 7A - Correlation of αCl vs. %Cl vs. CC. Figure 7B - Correlation of βCl vs. %Cl vs. CC. Figure 8A - Correlation of moisture variation vs. αCl. Figure 8B - Correlation of moisture variation vs. βCl. Figure 9 - Correlation of σCl vs. %Cl vs. CC. Figure 10 - Correlation of cement content vs. m1. PREFERRED EMBODIMENT OF THE INVENTION The present invention relates to a method for monitoring chloride content in porous materials. This method uses a voltammetric sensor, and its application in a medium such as concrete is a novel aspect.According to a preferred embodiment of the present invention, a rectangular potentiodynamic excitation signal, as shown in Figure 2, is applied to the sensor within the potential range where the reaction of interest occurs. After subjecting the sensor to an oxidative excitation pulse, a reductive relaxation pulse is applied to minimize alteration of the sensor surface (restoring its original metallic state), thereby increasing and ensuring its durability. According to another preferred embodiment of the present invention, a triangular signal, which may be of the type shown in Figure 1A, can be applied within the potential range where the reaction of interest occurs. In this case, it is the potential range where the formation / nucleation (oxidation) and reduction of chloride compounds with the metal of which the electrode is made takes place, as shown in Figure 1B.The voltammetric chloride sensor can be prepared from metals such as silver, gold, copper or lead, in which their ionic forms Ag. + , Cu + , Au + and Pb 2+ form poorly soluble compounds with the species to be detected (solubility Kps<10 -6The current intensity of the peaks related to the formation / reduction of the ionic compound formed with the chloride anion varies with the concentration in the medium and due to the high reactivity of the cations generated during the electrochemically induced oxidation of the metallic electrode in the presence of chlorides. The chloride content of the electrolyte in which the sensor is embedded is estimated from the processing and analysis of the electrical current obtained as a response. Chloride Sensor Preparation: As an example, the fabrication of a chloride sensor using 99% pure metallic silver (90% purity is acceptable for any of the metals used) is described. The procedure for preparing the sensor with any of the other metals mentioned (silver, gold, copper, and lead) is similar.Electrodes can be manufactured using sheets or wires, which, as an example, are dimensioned as follows: • Sheets: 0.1 to 0.5 mm thick; 5 mm to 20 mm long; • Wires: diameters between 0.1 mm and 2 mm; lengths between 5 mm and 20 mm. The effective electrode length, understood as the length in contact with the concrete, will depend on the porosity and electrical resistivity of the material in which the sensor is embedded. Larger electrode surfaces will be used in concretes with lower porosity and higher electrical resistivity. Table 1 specifies some recommendations based on the water-accessible porosity of the concrete obtained through the standardized test UNE 83980. Table 1 Recommended wire electrode dimensions according to water-accessible porosity of the concrete: Sensor System Operation: The distribution of sensors within the reinforced concrete structure allows for the detection and quantification of chlorides and their accumulation within the concrete mass over time. It enables the estimation of the depth and rate of chloride advance at various points within the material mass. Depending on the system monitoring requirements, two different configurations are proposed. Single-Sensor Configuration: Figure 3A shows a single-sensor configuration. One electrode is embedded in a cylinder of electrically conductive material (such as graphite or stainless steel), which acts as an auxiliary measuring electrode. The electrical connection to the electrode is made inside the cylinder using a multi-strand cable with a Teflon coating or any other equivalent polymer in terms of its air and water porosity.Next, to protect the cable-electrode connection and electrically isolate the electrode from the electrically conductive cylinder, the cylinder is filled with epoxy resin. The electrically conductive cylinder, which acts as the auxiliary measuring electrode, must have an outer surface area at least 40 times the effective surface area of ​​the sensor electrode. This electrically conductive cylinder is the counter electrode (CE) of the Cl sensor system. The electrical connection to the CE is made using multi-strand cable with Teflon insulation; the joint between the cable and the auxiliary conductive cylinder is protected with epoxy resin. Multisensor configuration: This configuration allows the same area to be monitored by several sensors, thus increasing the reliability of the estimation result. Figure 3B shows a compact multisensor system.Embedded within the electrically conductive cylinder are n sensors of the specified metals, in the same way as with a single electrode. The different wire sensors are positioned at known and fixed distances from each other. Measuring the detected concentration over time allows the chloride front velocity to be determined and its depth to be estimated. If the depth at which the reinforcement is located is known, estimating the initiation time from which depassivation of the structure's reinforcement occurs, and therefore the start of the reinforcement corrosion processes, becomes a simple calculation. Electrochemical Techniques Applied: In the sensor system, the electrochemical technique used can be cyclic voltammetry in the form of a triangular wave or potential step voltammetry, where the excitation signal is a rectangular wave.A dimensionless parameter, independent of temperature variations, is obtained that is directly correlated with the amount of chlorides present in the concrete pore solution. Rectangular wave signal: For the rectangular wave signal: the applied rectangular signal is characterized by the fact that during half a period the potential is ΔE. t=nThe pulse is ≠0 (excitation pulse) during one half of the period and 0 during the other (relaxation pulse). In this case, to determine the presence of the chloride anion, quantify it, and minimize errors caused by variations in electrical resistivity due to changes in the material's moisture content, two types of signals can be applied depending on the nature of the phenomenon being evaluated. Two types of signals can be applied based on their frequency. Excitation signal for non-faradic processes, related to the accumulation of charges at the sensor / material interface, is applied at high frequencies (between 25 Hz and 100 Hz). Excitation signal for faradic processes, related to the oxidation and reduction phenomena occurring on the sensor surface, is applied at low frequencies (between 1 Hz and 5 Hz). Pulses for inducing non-faradic processes: The signal frequency can range from 25 Hz to 100 Hz (high-frequency pulses).The maximum signal amplitude is 0.8 V and the minimum is -1 V. The signal amplitude (V) changes, in absolute value, at a rate of 50 mV / T, where T is the signal period. With this type of pulse, the electrical current response depends on the ionic mobility in the concrete and the charge accumulation in the double layer. Pulses to induce faradic processes: The signal frequency can range from 1 Hz to 5 Hz (low-frequency pulses). The maximum signal amplitude is 0.8 V and the minimum is -1 V. The signal amplitude changes, in absolute value, at a rate of 50 mV / T, where T is the signal period. With this type of pulse, the electrical current response depends primarily on the redox reactions that occur on the surface of the WE.With the aim of optimizing the investment costs required for monitoring large reinforced structures, the two excitation and measurement techniques can be applied using two different electronic and connection configurations: A traditional 3-electrode configuration, where the working electrode (WE) is the sensor and the counter electrode (CE) is either the electrically conductive cylinder shown in Figure 3 or the structure's own reinforcement. A reference electrode (REF) is also embedded in the concrete. A 2-electrode configuration, where the working electrode (WE) is the sensor and the electrically conductive cylinder shown in Figure 3 or the structure's own reinforcement is used as the counter electrode / pseudo-reference (CER). Figure 2 shows an example of the morphology of the applied excitation signals.The applied potential range encompasses the region where the formation and nucleation (oxidation) of AgCl and the reduction of this same ionic compound occur. The design and duration of the applied potential step sequence allow for the differentiation of metal oxidation processes that result in various types of oxides, hydroxides, or carbonates of the oxidized metals, and for distinguishing these species from the metal chlorides that form during the process. The cathodic potential step sequence aims to ensure complete regeneration of the sensor electrode surface and, therefore, that the response obtained in a subsequent measurement sweep is not affected by the sensor electrode's previous operating history. Figure 4 shows the accumulated charge curves obtained from the current density curves in Figure 5.It can be easily identified that the maximum charge correlates with the chloride concentration in the cement matrix, as it increases with increasing chloride concentration. Triangular Wave Signal: For cyclic voltammetry (CV) in a triangular waveform: The absolute scan rate can range from 20 mV / s to 70 mV / s. CV is applied in the potential range between 0.8 V and -1 V vs. SCE (saturated calomel electrode). Figure 1A shows an example of the morphology of the applied excitation signals. The applied potential range encompasses the region where the formation and nucleation (oxidation) of AgCl and the reduction of this same ionic compound occur. Figure 6 shows an example of the sensor's response to the described excitation type.A change in the morphology of the peaks associated with the reaction of the metal, in this case Ag, in the presence of chlorides is observed. The reduction peaks exhibit more stable behavior during tests in porous media (Figure 6, peaks P1, P2, and P3). In the absence of chlorides, during cathodic scanning, peak P1 is associated with the reduction of compounds formed by the metal with O2 and OH-. In the presence of chlorides, a new peak appears (P3, Figure 6), and peak P1 decreases. Since the formation of AgCl is favored over the formation of AgO and AgOH during oxidation, the reduction also favors the reduction of the most abundant product, resulting in the predominant peak P3. These peaks, P1 and P3, will be used to determine the chloride concentration using the sensor.With the aim of optimizing the investment costs required for monitoring large reinforced structures, the excitation and measurement technique can be applied using two different electronic and connection configurations: A traditional 3-electrode configuration, where the working electrode (WE) is the sensor and the counter electrode (CE) is either the electrically conductive cylinder shown in Figure 3 or the structure's own reinforcement. A reference electrode (REF) is also embedded in the concrete. A 2-electrode configuration, where the working electrode (WE) is the sensor and the electrically conductive cylinder shown in Figure 3 or the structure's own reinforcement is used as the counter electrode / pseudo-reference (CER).Data Processing and Chloride Estimation for the Non-Faradic Square Wave Signal. The average value of the ionic circulation resistance (Rs) of the excitation pulses for the non-Faradic signal is obtained. To do this, the value of the electrical current at time 0 of each of the high-frequency excitation pulses is determined by extrapolation, and by applying Ohm's law, the value of Rs is calculated for each pulse, using an analogy to the simple Randles circuit for electrochemical systems. The arithmetic mean of this parameter is then obtained from all the values. To obtain an Rs value related to chloride content and changes in humidity, the Rs value is normalized with respect to the day of commissioning when chlorides have not yet penetrated the concrete (Rs at time 0 = 0, Rs ≥ 0). According to the previous proposal, the correlation parameter, called αCl, is defined according to Eq. 1. where: t: is the time at which the trial is performed, α Cl : is the dimensionless parameter defined from the sensor response with the pulse signal for the induction of non-faradic processes, Rs t : is the value of the ionic circulation resistance of the system at the time t in which the test is performed (Ω), Rs0: is the value of the ionic circulation resistance of the system at the time of the start of exposure (Ω). The coefficient α Cl This is directly related to the chloride concentration, as demonstrated below. The resistance presented by the electrolyte (concrete pore solution) between WE and CE to the passage of ions is known as Rs, this resistance being the sum of as many resistances as there are analytes in the electrolyte. In the pore dissolution of concrete, under “normal” conditions the predominant anion is OH-, so the rest of the ionic conduction branches can be disregarded (Eq.3) Eq.3 In the case of the presence of chlorides, the ionic conduction branch associated with this anion must be considered (Eq. 4). 1 1 1^^^^ + ^^^^= + = ^^^^ ^^^^ ^^^ ^^^^ ^^^^^^^^ ^^^^Eq.4 ^^^^ ^^ ^^^^ ^ ^^^^ · ^^^^ Substituting Eq.3 and Eq.4 into Eq.1 and simplifying, we obtain ^^^^ ^^ ^^^^ = ^^^^ Eq.5 ^^^^ ^^^^ Conductance, the inverse of resistance, has a direct linear relationship with the ion concentration. Therefore, using this relationship in Eq. 5, we arrive at the expression that relates the defined parameter α. Cl with the chloride concentration. [^^^^−] = ^^^^^^ · ^^ · [^^^^ −Eq. 6, where K is a constant and [OH-], if there have been no pH variations, will also remain constant. Data processing and chloride estimation for the faradic rectangular wave signal. The system's response to the low-frequency faradic pulse rectangular wave signal is processed through the accumulated charge density. The charge is obtained from the integral of the intensity function with respect to time (Eq. 7). This integral can be solved geometrically, considering that the integral is equal to the area under the curve (Eq. 8). The accumulated charge curve is defined at each time step according to Eq. 9. Dividing this value by the sensor area yields the accumulated charge density. ^^^^^^^^^^^^^^^^^^^^ = ^^^^ + ^^^^−1 Eq.9where: Q: is the accumulated charge (C), i: is the electric current intensity (A), dt: is the time differential, qn: is the charge for an instant of time dt (C), Qaccumulated: is the charge accumulated for a time (C). To obtain a value of maximum accumulated charge due only to the reactions in which Chloride anions are involved, the Qmax accumulated is normalized with respect to the value obtained when chloride penetration has not yet occurred in the concrete (maximum accumulated charge at texposition=0, Qaccumulated MAX 0). Furthermore, considering that the auxiliary electrode and the reference will not always be at the same distance from the working electrode in all tests and for all electrodes, the cell constant will vary from case to case, so to compensate for this variation the difference defined above is divided by the maximum charge accumulated at texposition=0.According to the above, the correlation parameter β. cl It is defined according to Eq. 19. ^^^^^^^^^^^^^^^ − ^^^^^^^^^ ^^Á^^ ^^^^^^^^^^^^^^^^^ ^^Á^^ 0 ^^^^ = ^^ ^^^^^^^^^^^^^^^^^^ ^^Á^^ 0where: Q acumulada MÁX : is the maximum accumulated load value for a chloride-containing environment exposure period t, Q acumulada MÁX 0 : is the maximum accumulated charge value when chloride penetration has not yet occurred. The coefficient β ClThis is directly related to the chloride concentration in the concrete pore solution, as demonstrated below. The charge is defined according to Eq. 7, where i is the sum of all the faradic currents produced by the reaction of the electrode with different ions contained in the electrolyte, which can be expressed according to Eq. 11, the Cottrell equation. This equation expresses the variation of the intensity with respect to time for a given ion, as a function of its diffusion coefficient (D) and its concentration (C). 1 ^^ · ^^ · ^^ · ^ 2^^(^^) = ^ · ^^ ^ Eq.11 √ ^ · ^^ where: n is the number of electrons in the reduction or oxidation reaction of the analyte, F is the Faraday constant (96485 C / mol), S is the area of ​​the flat electrode in cm 2 , t is the time in seconds, in the trial. Therefore, solving the integral: In equation Eq. 12, the subscript 'a' refers to all ionic species contained in the electrolyte that react with the electrode during the test, and 'm' refers to the test duration over which the integral is performed. The diffusion coefficient and analyte concentration are fixed for a given exposure time to the environment. The maximum accumulated charge will occur at a specific test time (tQMAX). Therefore, the maximum accumulated charge can be expressed as: Eq. 13 Furthermore, in the case of chlorides, the sum n a ·D a 1 / 2 ·C a , it can be broken down by separating the term referring to the Cl- contained in the pore solution of the concrete. Eq.14 is substituted into Eq.19 and the common factor is obtained in both the numerator as in denominator, [2 · ^^1 / 2] ^^ ^^^^^^^^ 0, so that the terms cancel out. Eq.15 The chloride concentration at time 0 is equal to 0 (^^^^^^−0 = 0), and the concentration of the remaining analytes is assumed to be constant over time. ^^ ^^^^ = Eq.16 To put it simply: Considering that the diffusion coefficients will remain practically constant with exposure time, as well as the concentration of the other analytes other than the chloride anion, then ^^ ^^^^− ·^^ ^^^^− 1 will be equal to a constant that we will call K2. And Eq.17 ∑^^≠^^^^− (^^^^·^^^^ 2·^^ ^^ ) It becomes: ^^^^^^ = ^^2^^^^^^− Eq.18 where K2 is a constant for a given concrete, since it depends on the rest of the analytes contained in the pore solution of the concrete and C ClThe concentration of Cl- at a given exposure time. Data processing and chloride estimation for the triangular signal. The system's response to the triangular signal is processed through the current density (j) of the main reduction peak, where the main peak is defined as the one with the highest absolute value of j. In the example in Figure 6, this would be peak P1 for the chloride-free condition and peak P3 for the chloride-containing condition. The electrical current density (j) is defined as the electrical current (i) normalized by the effective surface area of ​​the sensor, where i is the sum of all the faradic currents produced by the electrode's reaction with different ions contained in the electrolyte. In the case where chlorides are present, Figure 6 shows an overlap with peaks due to other reactions (P2).To obtain a peak current density value due solely to reactions involving chloride anions, j is normalized to the value obtained when chloride penetration has not yet occurred in the concrete (peak current density of the main reduction, or the highest absolute value at texposition=0, j0). Since the auxiliary and reference electrodes will not always be at the same distance from the working electrode in all tests and for all electrodes, the cell constant will vary from case to case. To compensate for this variation, the previously defined difference is divided by the maximum accumulated charge value at texposition=0. Accordingly, the correlation parameter Cl is defined according to Eq. 19. where: j t : is the peak electric current density in reduction for a time from the start of exposure t (A / cm 2), j0: is the peak maximum electric current density in reduction for a time from the start of exposure 0 (A / cm 2 ). The coefficient σ Cl This is directly related to the chloride concentration in the concrete pore solution, as demonstrated below. The peak current in cyclic voltammetry can be expressed according to Eq. 20, the Randles-Sevcik equation. In this equation, the peak electric current is expressed with respect to the number of electrons in the redox reaction (n), the area of ​​the working electrode (S), the diffusion coefficient for the electroactive species (D), the scan rate (v), and the concentration of electroactive species (C). Thus, jt and j0 are defined as follows: In equations Eq. 22 and Eq. 23, the subscript a refers to all ionic species contained in the electrolyte that react with the electrode during the test, and t refers to the exposure time. The diffusion coefficient and the analyte concentration are fixed for a given exposure time in the environment. Eq. 22 and Eq. 23 are substituted into Eq. 19, and the terms that remain constant are canceled (vy 2.69·10 5 ). The chloride concentration at time 0 is considered to be 0 (^^^^^^−0 = 0), and the concentration of the rest of the analytes is assumed to be constant over time. To put it simply: Considering that the diffusion coefficients will remain practically constant with exposure time, as well as the concentration of the other analytes other than the anion 3 1 ^^ ^^^^ − ^^ 2 ·^^ ^^^^ − ^^2 chloride, then 3 1 will be equal to a constant that we will call K2. And the Eq. ∑^^≠^^^^− (^^^^0 2·^^ ^^0 2·^^ ^^0 ) 17 becomes: ^^^^^^ = ^^3^^^^^^− ^^ Eq.27 where K3 is a constant for a given concrete, since it depends on the other analytes contained in the pore solution of the concrete and C Cl the concentration of Cl- at a given exposure time. Example 1 Estimation and prediction model of the amount of chlorides in concrete (rectangular signal) As an example, Figure 7 shows, through empirical results obtained with an Ag sensor, a single-sensor type with a 2cm long and 1mm diameter wire, that the two defined parameters have a direct linear relationship with the percentage of chlorides, referred to the amount of cement per m 3 of concrete, contained in the concrete, showing R values 2greater than 0.92 (Figure 7). The free chloride content in the concrete was obtained according to the test specified in the RILEM TC 178-TMC standard. For obtaining the αCl parameter, in the results presented in Figure 7, the signal frequency used was 25 Hz (period (T) 40 ms). In the case of obtaining the βCl parameter, the signal frequency used was 5 Hz (period (T) 200 ms). The tests were carried out with the sensor embedded in two types of concrete: CC1, concrete with a water-cement ratio of 0.8, without additives, with a quantity of cement per m³ 3 of 255 kg concrete. CC2, concrete water cement ratio 0.6, without additives, with a quantity of cement per m 3 of 340 kg concrete. Since the percentage of chlorides is related to the amount of cement per m 3For concrete (CC), the slope of the fitted line will vary with CC, as shown in the graphs in Figure 7. Furthermore, the parameters αCl and βCl also vary with the concrete moisture content (Figure ). Therefore, the estimation model is based on two linear functions, one for each parameter, Eq. 28 and Eq. 29, where the estimated value is the percentage of chlorides as a function of the amount of cement per m³. 3of concrete. The parameters m1, m2, n1, and n2 will be constant for a specific amount of cement and humidity conditions. These parameters must be corrected based on the humidity and the amount of cement using empirical equations. %^^^^^^^^ = ^^1 · ^^^^^^ + ^^1 Eq.28%^^^^^^^^ = ^^2 · ^^^^^^ + ^^2 Eq.29 Furthermore, a prediction model is also defined for estimating the time it will take for chlorides to reach a certain percentage at the sensor height. The inputs to this model are the parameters obtained from the sensor response and the time it takes for chlorides to reach the sensor height. The equations governing it have been obtained empirically and through dimensional analysis.Example 2 Estimation and prediction model of the amount of chlorides in concrete (triangular signal) As an example, Figure 9 shows, through empirical results obtained with an Ag sensor, a monosensor type of 2cm long wire and 1mm in diameter, that the defined parameter has a direct linear relationship with the percentage of chlorides, referred to the amount of cement per m. 3 of concrete, contained in the concrete, showing R values 2greater than 0.92 (Figure 9). The free chloride content in the concrete was obtained according to the test specified in the RILEM TC 178-TMC standard. To obtain the σCl parameter, cyclic triangular wave voltammetry was used as the excitation signal (Figure 1A), with a sweep speed of 20 mV / s. The triangular wave voltammetry was applied in the potential range between 0.8 V and -1 V vs. SCE. The tests were performed with the sensor embedded in five types of concrete: CC1, concrete with a water-cement ratio of 0.9, without additives, with a quantity of cement per m³ 3 of 225 kg concrete. CC2, concrete water cement ratio 0.8, without additives, with a quantity of cement per m 3 of 250 kg concrete. CC3, concrete water cement ratio 0.6, without additives, with a quantity of cement per m 3 of 315 kg concrete. CC4, concrete water cement ratio 0.5, without additives, with a quantity of cement per m 3of 385 kg concrete. CC5, concrete water cement ratio 0.4, without additives, with a quantity of cement per m 3 of 490 kg of concrete. Since the percentage of chlorides is related to the amount of cement per m 3 For concrete (CC), the slope of the fitted line will vary with CC, as shown in the graphs in Figure 10. Therefore, the estimation model is based on a linear function Eq., where the estimated value is the percentage of chlorides as a function of the amount of cement per m 3 of concrete. The parameter m1 will be constant for a specific amount of cement and moisture conditions. This parameter must be corrected according to the moisture and amount of cement using empirical equations. %^^^^^^^^ = ^^3 · ^^^^^^ Eq.30 It is demonstrated from the results of the study that the relationship of m1 with CC follows a function of the type ^^ =^^^^+^^ where a, b, c, and d are constants, and their values ​​can be obtained by fitting the actual data to the theoretical function (Figure 10). In the case at hand, the equation for m1 as a function of CC is shown in Eq. 31 ^^ 0.67 · ^^^^ − 88.17 3 =2.17 · ^^^^ − 460.69Eq.31 Furthermore, a predictive model is also defined for estimating the time it takes for chlorides to reach the sensor at a given percentage. The inputs to this model are the parameters obtained from the sensor's response and the time it takes for chlorides to reach the sensor's height. The equations governing this model have been obtained empirically and through dimensional analysis. Conclusions The procedure according to the present invention offers the following advantages: - The sensor is capable of detecting the presence of chlorides in concrete exposed to a NaCl concentration 0.1 m or higher. - The sensor's sensitivity is high, allowing it to differentiate the presence of chlorides when their percentage in the concrete relative to the amount of cement is equal to or greater than 0.01%.- The parameter defined from the sensor response, used to estimate the presence of chlorides, has been verified to be independent of temperature. - The parameter defined from the sensor response, used to estimate the presence of chlorides, has been verified to be dependent on the concrete's moisture content, which is taken into account in the model. - The fit between estimated and calculated values ​​yields a slope of 0.931 and an Y-intercept of 0.039, with an acceptable R² value (0.90). - The sensor is capable of estimating the time at which a specific chloride percentage will occur with an error typically less than five percent.

Claims

CLAIMS 1. A method for monitoring chloride content in porous materials, characterized in that: - a voltammetric sensor comprising a metal electrode is used, wherein the electrode metal forms sparingly soluble ionic compounds with chlorides; - a potentiodynamic excitation signal is applied to the electrode, producing an oxidation-reduction process that favors the formation of the sparingly soluble ionic compounds; - the formation of the sparingly soluble ionic compounds produces an electrical current intensity response that is related to the chloride content in the monitored material.

2. A method according to claim 1, characterized in that the porous materials are made of concrete.

3. A method according to any of the preceding claims, characterized in that the electrode metal forms ionic compounds with chlorides of solubility Kps ≤ 10 -64. A process according to any of the preceding claims, characterized in that the electrode metal is selected from the group comprising Ag, Cu, Au, or Pb.

5. A process according to claim 4, characterized in that the electrode metal is Ag.

6. A process according to any of the preceding claims, characterized in that the electrode metal has a purity ≥ 90%, preferably 99%.

7. A process according to any of the preceding claims, characterized in that the metal electrode is in the form of a sheet or a wire.

8. A process according to claim 7, characterized in that the sheet has a thickness of between 0.1 and 0.5 mm and a length of between 5 and 20 mm; and the wire has a diameter of between 0.1 and 5 mm and a length of between 5 and 20 mm.

9. A method according to any of the preceding claims, characterized in that the potentiodynamic excitation signal is applied to the electrode within a potential range that produces the oxidation and reduction reaction of the metal / Cl- ionic compound.

10. A method according to claim 9, characterized in that the potential range has a maximum signal amplitude of 0.8 V and a minimum amplitude of -1 V.

11. A method according to any of the preceding claims, characterized in that the applied potentiodynamic excitation signal is rectangular or triangular in shape.

12. A method according to claim 11, characterized in that, for the rectangular-shaped signal, pulses are applied to induce non-faradic processes with a signal frequency between 25 and 100 Hz. 13.A method according to claim 11, characterized in that pulses are applied to the rectangular-shaped signal to induce faradic processes with a signal frequency of between 1 and 5 Hz.

14. A method according to any of claims 12-13, characterized in that the signal amplitude changes at a rate of 50 mV / T, where T is the signal period.

15. A method according to claim 11, characterized in that a sweep speed of between 20 mV / s and 70 mV / s is applied to the triangular-shaped signal.

16. A method according to any of the preceding claims, characterized in that the voltammetric sensor is used in a single-sensor configuration comprising an electrode embedded in a cylinder of electrically conductive material. 17.A method according to any of claims 1-15, characterized in that the voltammetric sensor is used in a multi-sensor configuration comprising n electrodes embedded in a cylinder of electrically conductive material.

18. A method according to any of claims 1-12 or 14, characterized in that excitation pulses for the non-faradic signal are applied to the electrode. - The chloride content is calculated based on a correlation parameter αCl according to the following formula: 1 − 1 ^^^^ ^^ ^^ ^^^^0 ^^^^ = Eq.1 1 ^^^^0where: t: is the time at which the test is done, Rs t: is the value of the ionic circulation resistance of the system at time t in which the test is performed (Ω). Rs0: is the value of the ionic circulation resistance of the system at the start of exposure (Ω).

19. Method according to any of claims 1-11 or 13-14, characterized in that - excitation pulses for the faradic signal are applied to the electrode and - the chloride content is calculated based on a correlation parameter β Cl according to the following formula: where: Q accumulated MAX: is the maximum accumulated charge value for a chloride-containing environment exposure period t. Q accumulated MAX 0: is the maximum accumulated charge value when chloride penetration has not yet occurred.

20. A method according to any of claims 1-11 or 15, characterized in that - triangular excitation pulses are applied to the electrode and - the chloride content is calculated based on a correlation parameter ^^ Cl according to the following formula: ^ ^ ^^ ^^ − ^^0 ^^^^ = Eq.209 ^^0where: jt: is the electric current density of the maximum peak in reduction for a time from the start of exposure t (A / cm 2 ) j0: is the peak electric current density in reduction for a time from the start of exposure 0 (A / cm 2 ).

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